The present invention relates to heat-shrinkable polyester films and particularly to heat-shrinkable polyester films suitable for label use. More specifically, it relates to heat-shrinkable polyester films for full labels on bottles, particularly for full labels on polyethylene terephthalate (PET) bottles, which will cause only rare occurrence of wrinkles, shrinkage spots and strains by heat shrinkage.
For heat-shrinkable films, particularly heat-shrinkable films for labels on the barrels of bottles, there have mainly been used films made of polyvinyl chloride, polystyrene or other resins. However, for polyvinyl chloride, the evolution of a chlorine-containing gas in its incineration for disposal has become a problem, and for polyethylene, there is a problem that printing is difficult. Further, in the collection and recycling of PET bottles, labels of resins other than PET, such as polyvinyl chloride or polyethylene, should be separated. Therefore, attention has been paid to heat-shrinkable polyester films involving these problems.
In recent years, for recycling of PET bottles, colored bottles are not suitable for reuse, and therefore, many alternative plans have been studied. Among them is a method of using a non-colored bottle and allowing a colored label to shrink over the bottle.
However, when used as full labels on bottles, the conventional heat-shrinkable films may sometimes cause a problem in shrinkage finish. In the case of full labels on narrow-mouthed bottles having a large difference in bottle diameter between the mouths and the barrels, particularly such as beverage bottles, the conventional heat-shrinkable films exhibit insufficient shrinkage at the upper neck portions of the bottles. The heat-shrinkable films to be used for full labels on such bottles should have heat-shrinkage characteristics such as high shrinkability.
Thus in the case of full label use on bottles, the conventional heat-shrinkable films have insufficient performance.
For example, the films disclosed in JP 2000-169601 A exhibit high shrinkage, but have not yet reached a level requiring shrinkage finish.
The present invention, which can solve the above problems, has been made for the purpose of providing heat-shrinkable polyester films for full labels on bottles, particularly for full labels on PET bottles, which films are particularly used for full labels on PET bottles requiring excellent finish in appearance and will cause only rare occurrence of wrinkles, shrinkage spots and strains by shrinkage.
Thus the present invention is directed to heat-shrinkable polyester films, each having a heat shrinkability of 5-60% after treatment in hot water at 70xc2x0 C. for 5 seconds and 65-95% after treatment in hot water at 85xc2x0 C. for 5 seconds, both in the main shrinkage direction of the film, and 10% or lower after treatment in hot water at 85xc2x0 C. for 5 seconds in a direction perpendicular to the main shrinkage direction of the film, the heat-shrinkage stress in the main shrinkage direction of the film being 13 MPa or smaller, and the film haze being lower than 10%, thereby attaining the above purpose.
The dicarboxylic acid components constituting polyesters used in the present invention may include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and o-phthalic acid; aliphatic dicarboxylic acids such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid; and alicyclic dicarboxylic acids.
When aliphatic dicarboxylic acids (e.g., adipic acid, sebacic acid, decanedicarboxylic acid) are contained, their contents may preferably be lower than 3 mol %. For heat-shrinkable polyester films obtained by the use of polyesters containing these aliphatic dicarboxylic acids at 3 mol % or higher, their film stiffness in the high-speed fitting is insufficient.
The polyesters may preferably contain no three or more functional polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid, their anhydrides). For heat-shrinkable polyester films obtained by the use of polyesters containing these polycarboxylic acids, their desired high shrinkability can hardly be attained.
The diol components constituting the polyesters used in the present invention may include aliphatic diols such as ethylene glycol, propanediol, butanediol, neopentyl glycol and hexanediol; alicyclic diols such as 1,4-cyclohexanedimethanol; and aromatic diols.
The polyesters used in the heat-shrinkable polyester films of the present invention may preferably be those having glass transition temperatures (Tgs) adjusted to 60-75xc2x0 C. by the incorporation of at least one diol of 3-6 carbon atoms (e.g., propanediol, butanediol, neopentyl glycol, hexanediol).
For the purpose of obtaining heat-shrinkable polyester films exhibiting particularly excellent shrinkage finish, neopentyl glycol may preferably be used as one of the diol components.
The polyesters may preferably contain neither diols of 8 or more carbon atoms (e.g., octanediol), nor three or more functional polyhydric alcohols (e.g., trimethylolpropane, trimethylolethane, glycerin, diglycerin). For heat-shrinkable polyester films obtained by the use of polyesters containing these diols or polycarboxylic acids, their desired high shrinkability can hardly be attained.
The polyesters may preferably contain none of diethylene glycol, triethylene glycol, and polyethylene glycol, if possible. In particular, diethylene glycol may easily be formed as a by-product component in the polymerization of polyesters and therefore may easily be contained in the polyesters. For the polyesters used in the present invention, the diethylene glycol contents may preferably be lower than 4 mol %.
For the heat-shrinkable films of the present invention, polyesters meeting the above conditions may be used alone, or two or more such polyesters may be used in admixture.
When two or more polyesters are used in admixture, the acid component contents and the diol component contents of the mixed polyesters refer to the contents, relative to the total amount of all acid components and the total amount of all diol components, respectively, both of which are contained in the mixed polyesters, independently of whether or not transesterification has been carried out after the mixing.
The mode for using two or more polyesters in admixture may include, for example, the following cases:
1. PET/copolymerized PES
2. copolymerized PES/copolymerized PES
3. PET/copolymerized PES/copolymerized PES
Further, to improve the self-lubricating properties of heat-shrinkable polyester films, inorganic lubricants such as titanium dioxide, fumed silica, kaolin and calcium carbonate; or organic lubricants such as long-chain fatty acid esters may preferably be added. The heat-shrinkable polyester films of the present invention may further contain, if necessary, additives such as stabilizers, colorants, antioxidants, defoamers, antistatic agents and ultra-violet light absorbers.
For the heat-shrinkable polyester films of the present invention, amorphous polyesters may preferably be incorporated to attain high shrinkability. In addition to amorphous polyesters, polyester elastomers may preferably be incorporated at 5-24 wt %, more preferably at 10-24 wt %, still more preferably at 15-24 wt %, and particularly preferably 17-24 wt %. When the amounts of polyester elastomers are lower than 5 wt %, low temperature shrinkability is hardly expressed, and therefore, failures in shrinkage, such as wrinkles, will easily occur. In contrast, when the amounts of polyester elastomers are higher than 24 wt %, high shrinkability is hardly expressed, and therefore, failures in shrinkage, such as insufficient shrinkage at the necks of bottles, will easily occur.
Further, it has also been found that shrinkage stress in the shrinkage by heating may easily be reduced by the incorporation of polyester elastomers. Not too great amounts of amorphous polyesters incorporated bring about an easy reduction of shrinkage stress in the shrinkage by heating.
The polyester elastomers used in the present invention refer to polyester block copolymers composed of constituent units as follows: high melting point crystalline polyester segments (hard segments) and low melting point soft polymer segments having molecular weights of 400 or higher (soft segments) wherein the melting points of high molecular weight polymers formed only by high melting point crystalline polyester segment constituent components are 200xc2x0 C. or higher and the melting points or softening points when measured only for the low melting point soft polymer segment constituent components are 80xc2x0 C. or lower.
The high melting point crystalline polyester segment constituent components exhibit melting points of 200xc2x0 C. or higher when fiber forming high molecular weight polymers are formed from these constituent components. Examples thereof may include polyester segments composed of residues of aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 1,5-naphthalenedicarboxylix acid and 2,6-naphthalenedicarboxylic acid; and residues of aliphatic, aromatic or alicyclic diols such as ethylene glycol, propylene glycol, tetramethylene glycol, pentamethylene glycol, 2,2-dimethyltrimethylene glycol, hexamethylene glycol, decamethylene glycol, p-xylene glycol and cyclohexanedimethanol.
There can also be mentioned copolymerized polyester segments using two or more acids as described above and/or two or more glycols as described above.
The low melting point soft polymer segment constituent components having molecular weights of 400 or higher refer to those which exhibit a substantially amorphous state among the polyester block copolymers and have melting points or softening points of 80xc2x0 C. or lower when measured only for these segment constituent components. Their molecular weights may usually be 400-8000, preferably 700-5000.
The ratios of low melting point soft polymer segment constituent components in the polyester block copolymers may preferably be 5-80 wt %. Particularly preferred ratio is 15-50 wt %.
For the low melting point soft polymer segment constituent components used in the present invention, particularly preferred are polymers of cyclic esters such as poly-xcex5-caprolactone. Further preferred are those which contain polybutylene terephthalate components.
The above polyesters can be prepared by polymerization according to the conventional methods; for example, the polyesters can be obtained direct esterification method in which dicarboxylic acids are directly reacted with diols or transesterification method in which dicarboxylic acid dimethyl esters are reacted with diols. The polymerization may be carried out in either of batch or continuous manner.
For the heat-shrinkable polyester films of the present invention, the heat shrinkability as calculated from the values of side length before and after shrinkage by treatment in hot water under no load according to the equation: heat shrinkability=((side length before shrinkagexe2x88x92side length after shrinkage)/side length before shrinkage)xc3x97100 (%) should be 5-60%, preferably 5-50%, and more preferably 10-30%, after treatment in hot water at 70xc2x0 C. for 5 seconds, and 65-95%, preferably 65-75%, and more preferably 65-70% after treatment in hot water at 85xc2x0 C. for 5 seconds, both in the main shrinkage direction of a film, and 10% or lower, preferably 6% or lower, after treatment in hot water at 85xc2x0 C. for 5 seconds, in a direction perpendicular to the main shrinkage direction of the film.
When the heat shrinkability in the main shrinkage direction of a film is lower than 5% after treatment in hot water at 70xc2x0 C. for 5 seconds, labels made of such a film exhibit insufficient shrinkage at low temperatures, requiring the rise in temperature for shrinkage, which is not preferred. In contrast, when the heat shrinkability is higher than 60%, labels made of such a film cause jumping by heat shrinkage, which is also not preferred.
As described above, the heat shrinkability in the main shrinkage direction of a film should be 65-95% after treatment in hot water at 85xc2x0 C. for 5 seconds. When the heat shrinkability is lower than 65%, labels made of such a film cause insufficient shrinkage at the mouth portions of bottles, which is not preferred. In contrast, when the heat shrinkability is higher than 95%, labels made of such a film have a possibility of causing jumping because of their still having shrinkability after heat shrinkage, which is also not preferred.
The heat shrinkage stress in the main shrinkage direction of a film should be 13 MPa or smaller at 90xc2x0 C. This is because the present inventors have extensively studied and then found that there is a correlation between the heat shrinkage stress and the shrinkage finish. In other words, this seems because if the shrinkage stress is low, shrinkage gradually occurs and the film uniformly shrinks in each portion. The heat shrinkage stress in the main shrinkage direction of a film may preferably be 10 MPa or smaller. When the heat shrinkage stress is larger than 13 MPa, labels made of such a film easily cause jumping, strains and other defects by heat shrinkage, which is not preferred.
For adjusting the heat shrinkage stress in the main shrinkage direction of a film to 13 MPa or smaller at 90xc2x0 C. in this manner, it is preferable, for example, that the film is not so much stretched at a high ratio. At this time, if the film does not stretched at so higher temperatures, higher shrinkability can be obtained. Further, setting lower a glass transition temperature of a heat shrinkable film is preferred for obtaining shrinkability at low temperatures.
The difference of heat shrinkability (xcex94HS) in the main shrinkage direction of a film between after treatment in hot water at 80xc2x0 C. for 5 seconds and after treatment in hot water at 70xc2x0 C. for 5 seconds may preferably be 60% or smaller, more preferably 55% or smaller. When the difference of heat shrinkability is smaller than 60%, rapid heat shrinkage hardly occurs, so that labels made of such a film cause only rare occurrence of the wrinkles, shrinkage spots, strains and other defects by heat shrinkage.
For the heat-shrinkable polyester films of the present invention, the film thickness is not particularly limited, but may preferably be 10-200 xcexcm, more preferably 20-100 xcexcm, as the heat-shrinkable films for labels.
The films may preferably have a haze of lower than 10%, more preferably lower than 9%. When the film haze is lower than 10%, the coloring of labels after printing is improved, which is preferred.
The process for producing the heat-shrinkable polyester films of the present invention will be explained below by a specific example; however, it is not limited to this example.
The polyester raw materials used in the present invention are dried with a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer, and melt extruded into a film shape at a temperature of 200-300xc2x0 C. In the extrusion, any of the conventional methods may be employed, including T-die method and tubular method. After the extrusion, rapid cooling provides an unstretched film.
The resulting unstretched film is then stretched at a ratio of 3.0 or higher, preferably 3.5 or higher, in the transverse direction (i.e., the direction perpendicular to the direction of extrusion) at a temperature of (Tgxe2x88x925xc2x0 C.) or higher but lower than (Tg+15xc2x0 C.). Stretching at a temperature of Tg or higher but lower than (Tg+10xc2x0 C.) is preferred in that both high shrinkability and low shrinkage stress are attained.
When stretching is carried out at a temperature of lower than (Tgxe2x88x925xc2x0 C.), the values of heat shrinkability as the claimed conditions in the present invention can hardly be obtained and the resulting film has deteriorated transparency, which is not preferred.
When stretching is carried out at a temperature of (Tg+15xc2x0 C.) or higher, the resulting film has insufficient film stiffness for high-speed fitting and remarkably deteriorated thickness distribution, which is not preferred.
The stretched film is then heat treated, if necessary, at a temperature of 65-100xc2x0 C. to give a heat-shrinkable polyester film.
The method of stretching may involve uniaxial stretching only in the transverse direction (TD) with a tenter, in which case the film can additionally be stretched in the machine direction (MD) to attain biaxial stretching. Such biaxial stretching may be achieved by any of the sequential or simultaneous biaxial stretching method, and the film may further be stretched, if necessary, in the machine or transverse direction.
For attaining the purpose of the present invention, the transverse direction (i.e., the direction perpendicular to the direction of extrusion) is practical as the main shrinkage direction of a film; therefore, the above explanation is for an example of the film formation when the main shrinkage direction of the film is taken as the transverse direction. However, the film formation in which the main shrinkage direction of the film is taken as the machine direction (i.e., the direction of extrusion) can also be carried out substantially in the same manner as described above, except that the direction of stretching is turned 90 degrees around the line perpendicular to the film surface.
For the heat-shrinkable polyester films of the present invention, the thickness distribution of a film as calculated from the values of film thickness by the following equation:       Thickness    ⁢          xe2x80x83        ⁢    distribution    =                                                                        Maximum                ⁢                                  xe2x80x83                                ⁢                thickness                            -                                                                          Minimum              ⁢                              xe2x80x83                            ⁢              thickness                                                  Average        ⁢                  xe2x80x83                ⁢        thickness              xc3x97    100    ⁢          xe2x80x83        ⁢          (      %      )      
may preferably be 6% or lower, more preferably 5% or lower.
The films having a thickness distribution of 6% or lower are easy to achieve the superposition of colors in the three-color printing carried out, for example, in the evaluation of shrinkage finish, whereas the films having a thickness distribution of higher than 6% are not preferred from the viewpoint of color superposition.
To make even thickness distribution in the heat-shrinkable polyester films of the present invention, an unstretched film may preferably be heated to a prescribed film temperature at a low air flow rate with a heat transmission coefficient of 0.0013 cal/cm2xc2x7secxc2x7xc2x0 C. (0.0054 J/cm2xc2x7secxc2x7K) or lower in the step of preheating to be carried out prior to the step of stretching when stretched in the transverse direction with a tenter.
In this case, preheating at an air flow rate of 12-16 m/sec through a nozzle is preferred even for a decrease in haze.
The equipment for preheating an unstretched film may include, for example, equipment provided with an inverter for controlling the flow rate of hot air to heat the film, making it possible to prevent a change in the air flow rate; and equipment using low-pressure steam at a pressure of 50 kPa or lower (5 kgf/cm2 or lower) as the heat source, making it possible to prevent a change in the temperature of hot air.
To prevent the internal heat evolution of a film involved in stretching to reduce the unevenness of film temperature in the width direction, the step of stretching may preferably be conditioned with a heat transmission coefficient of 0.0009 cal/cm2xc2x7secxc2x7xc2x0 C. (0.0038 J/cm2xc2x7secxc2x7K) or higher, more preferably 0.0011-0.0017 cal/cm2xc2x7secxc2x7xc2x0 C. (0.0046-0.0072 J/cm2xc2x7secxc2x7K).
When the air flow rate employed in the step of preheating corresponds to a heat transmission coefficient of higher than 0.0013 cal/cm2xc2x7secxc2x7xc2x0 C. (0.0054 J/cm2xc2x7secxc2x7K) or when the air flow rate employed in the step of stretching corresponds to a heat transmission coefficient of lower than 0.0009 cal/cm2xc2x7secxc2x7xc2x0 C. (0.0038 J/cm2xc2x7secxc2x7K), the film thus obtained is difficult to have even thickness distribution, so that it causes the deviation of patterns in the superposition of many colors when processed in the multi-color printing, which is not preferred.